Designed Peptide Analogues of the Potassium Channel Blocker ShK Toxin

Abstract

ShK toxin, a potassium channel blocker from the sea anemone Stichodactyla helianthus, is a 35-residue polypeptide cross-linked by 3 disulfide bridges. In an effort to generate truncated peptidic analogues of this potent channel blocker, we have evaluated three analogues, one in which the native sequence was truncated and then stabilized by the introduction of additional covalent links (a non-native disulfide and two lactam bridges), and two in which non-native structural scaffolds stabilized by disulfide and/or lactam bridges were modified to include key amino acid residues from the native toxin. The effect of introducing a lactam bridge in the first helix of ShK toxin (to create cyclo14/18[Lys14,Asp18]ShK) was also examined to confirm that this modification was compatible with activity. All four analogues were tested in vitro for their ability to block Kv1.3 potassium channels in Xenopus oocytes, and their solution structures were determined using 1H NMR spectroscopy. The lactam bridge in full-length ShK is well tolerated, with only a 5-fold reduction in binding to Kv1.3. The truncated and stabilized analogue was inactive, apparently due to a combination of slight deviations from the native structure and alterations to side chains required for binding. One of the peptide scaffolds was also inactive because it failed to adopt the required structure, but the other had a K(d) of 92 microM. This active peptide incorporated mimics of Lys22 and Tyr23, which are essential for activity in ShK, and an Arg residue that could mimic Arg11 or Arg24 in the native toxin. Modification of this peptide should produce a more potent, low molecular weight peptidic analogue which will be useful not only for further in vitro and in vivo studies of the effect of blocking Kv1.3, but also for mapping the interactions with the pore and vestibule of this K(+) channel that are required for potent blockade.